In the medieval era, most people had only a given name, such as John. To help differentiate, some were also known by the manor in which they resided � John became John of Cornwall Manor. This would then be added to by things such as trade � Edward the Metalsmith, Marcus the Carpenter or Jacob the Miller. In different countries or cultures, "the" would be replaced by "O' ", short for "of", or "Mc" or "Mac", or "Le" or "La".
Over the centuries these turned into the last names of today (just drop �the�). In this vein, if John had a son Charles, he might be know as Charles, John�s son (later known as Charles Johnson).